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The present invention relates to radio frequency systems, and particularly to the cancellation of harmonics generated in high power radio frequency amplifiers used for communications, electronic warfare, and other RF systems.
Managing harmonic frequency distortion in radio frequency (RF) amplifiers and transmission systems is always a prime consideration for system designers. Given that the most efficient RF power amplifiers are commonly class AB or class C amplifiers that operate on less than the complete small signal waveform, generating large harmonic distortion components, designers have to include hardware in RF systems that removes or suppresses the harmonic frequency content of the system output.
In fixed frequency systems the problem is easily managed by including filters at the amplifier output. Tuned frequency systems, and frequency-agile systems particularly, must be capable of filtering harmonics across broad ranges of frequency that change constantly. This most often requires a bank of filters having different cutoff frequencies that must be switched into and out of the system as the transmission frequency changes. Where the frequency changes quickly and often, switching these filters becomes more difficult and places restrictions on system performance related to the finite switching times and impulse responses of the filters.
The most efficient way to reduce the harmonic distortion of a generated RF signal is to suppress the generation of the harmonic distortion in the amplifier without trading away the efficiency of the broadband amplifier. The present invention is directed to controlling the harmonic content of the output of high power RF amplifiers for modulated RF systems.
One method of controlling harmonics in the output of a device is to predistort the input signal to the device. The predistortion creates frequency products at the input that, when passing through the non-linear amplifier device, combine with and cancel the normal harmonic output of the device. This is normally done at baseband or intermediate frequencies, predistorting the modulation signal (or an IF) before combining it with the final RF carrier wave. In this way, the harmonic distortion of the information signal may be controlled within the modulated signal output.
However, nothing in this process addresses the problem of the harmonic frequency components generated by an RF amplifier operating on the modulated RF carrier wave. These harmonics must be controlled at the output of the transmission system. The present invention is intended to control RF harmonic output to avoid the necessity for high power switched RF filters and the practical limitations that they impose on a tuned frequency transmission system.
The present invention is a harmonic cancellation system for broadband, high-power RF transmission systems. A transmission system constructed according to the invention is capable of amplifying a modulated RF waveform, performing direct digital sampling of the RF waveform, analyzing the harmonic content of the waveform, and generating a predistortion signal to feed back into the amplifier input to suppress the harmonics generated in the RF amplifier.
The invention comprises a high-power RF amplifier, a directional coupler for diverting a small portion of the amplifier output to a feedback controlled, harmonic cancellation processor, a high-speed digital sampling device, a digital signal processing device for analysis of the RF signal harmonic content, a digital waveform generator for generating a predistorted RF carrier signal, and a modulation input and control.
The harmonic cancellation system comprises a computer-based apparatus for cancelling output signal harmonics in a radio frequency transmission system, comprisinga radio frequency amplifier, a means for diverting a portion of the amplifier output as a feedback signal. The system further comprises a computer-controlled harmonic cancellation processor, said processor comprising an analog to digital converter for sampling the feedback signal from the diverting means, a digital signal processor for calculating the harmonic content of the amplifier output feedback signal, said digital signal processor comprising machine-readable program instructions for analyzing the feedback signal harmonic content and generating a predistorted RF waveform for cancelling the harmonics generated by the radio frequency amplifier, and a digital to analog converter to produce a predistorted RF waveform for input to the radio frequency amplifier.
The invention further comprises a digital signal processor having machine readable instructions for analyzing the harmonic content of the RF output signal and generating a predistorted RF signal for combining with the modulated RF carrier wave input into the RF amplifier. The instructions comprise a method for cancelling the harmonics comprising the steps: creating a matrix of real-time digital samples of a representative portion of an RF waveform produced by an amplifier, performing a fast Fourier transform on the samples of RF waveform to quantify the harmonic content of the waveform, comparing each measured harmonic magnitude and phase to an immediately previous magnitude and phase that is stored in memory for the same harmonic, comparing each harmonic magnitude and phase in the current predistorted waveform for input to the amplifier to an immediately previous magnitude and phase that is stored in memory for the same harmonic, calculating a new model transfer function for the amplifier using the results of the comparisons of the previous two steps, computing a new RF input waveform for the amplifier that is predistorted using the new model transfer function for the amplifier, generating a new RF input waveform with predistortion for input to the RF amplifier to cancel harmonics produced by the amplifier.